The present invention relates generally to a method for producing nitride ceramic powders, and more particularly to a combustion synthesis method thereof.
The family of nitride ceramic powders have the excellent properties such as high melting points, satisfactory hardnesses, excellent mechanical strengthes under high temperatures, and satisfactory chemical stabilities. Some even have excellent thermal conductivities. Therefore, they are widely applied in many fields. For example, silicon nitride is acknowledged as one of the best materials for engines, turbines and other high temperature applications, and aluminum nitride is very important in the applications of high-performance electronic substrate materials, optical lenses, cutting tools, heat sinks . . . etc.
Conventional methods for producing nitride ceramic powders include:
1) the direct nitridation method, e.g., ##STR1## 2) the carbothermal reduction and nitridation method, e.g., ##STR2## 3) the imide decomposition method, e.g., EQU SiCl.sub.4(aq) +6NH.sub.3(aq) .fwdarw.Si(NH).sub.2(s) +4NH.sub.4 Cl PA1 4) the vapor-phase synthesis method, e.g., EQU 3SiCl.sub.4(g) +4NH.sub.3(g) .fwdarw.Si.sub.3 N.sub.4(s) +12HCl.sub.(g) ; PA1 5) the combustion synthesis method. PA1 A) they take relatively much time; PA1 B) they consume relatively much energy; PA1 C) their conversions are relatively low; and PA1 D) the purities of their products are relatively low. PA1 A) its cost is relatively high; PA1 B) it involves relatively many steps; and PA1 C) it takes relatively much time. PA1 A) it consumes relatively much energy; PA1 B) the production rate is relatively low; and PA1 C) it is hard to collect the obtained powder. PA1 A) the reaction is relatively fast; PA1 B) it consumes much less energy than the aforementioned methods do; PA1 C) the production procedure is relatively simple; and PA1 D) mass production can be easily achieved. PA1 1) U.S. Pat. No. 4,459,363 entitled SYNTHESIS OF REFRACTORY MATERIALS discloses a method for synthesizing refractory nitride materials. The steps include mixing a metal azide with Ca or Mg and at least a stoichiometric amount of a metal oxide selected from the rare earth metals, the metals of the III-A, IV-A, III-B, and IV-B groups, or a mixture thereof, heating the resulting mixture until the ignition temperature is attained, and allowing the mixture to combust in a self-propagating manner to form a refractory nitride composition. PA1 2) U.S. Pat. No. 4,944,930 entitled SYNTHESIS OF FINE-GRAINED .alpha.-SILICON NITRIDE BY A COMBUSTION PROCESS discloses a method for the combustion synthesis of .alpha.-silicon nitride. The steps include dry-mixing silicon powder with an alkali metal azide, such as sodium azide, cold-pressing the mixture into any desired shape, or loading the mixture into a fused, quartz crucible, loading the crucible into a combustion chamber, pressurizing the chamber with nitrogen and igniting the mixture using an igniter pellet, and isolating the .alpha.-silicon nitride formed as a reaction product. PA1 3) The Japanese PAT. PUBLICATION NO. 64-76906 discloses a method for producing a nitride powder. The method includes steps of mixing a powdery metal and the metal nitride in an appropriate ratio to form a mixture, placing the mixture in a porous, refractory vessel surrounded by liquid nitrogen, and igniting the synthesis reaction by electrical heating to form a powdery nitride product. PA1 4) The Japanese PAT. PUBLICATION NO. 64-76905 discloses a method for producing nitride powders. The method includes steps of mixing a powdery metal and NaN.sub.3 (or other solid-state nitride, e.g., KN.sub.3, Ba.sub.3 N.sub.2 etc.) in an appropriate ratio to form a mixture, putting the mixture into a refractory vessel, placing an igniting agent on the top of the powdery mixture, placing the vessel with the mixture in an electrical to oven which is enclosed in a container filled with N.sub.2 gas (having a pressure smaller than 10 kg/cm.sup.2), heating the powdery Al and NaN.sub.3 in the oven, and igniting the igniting agent to start and complete the combustion synthesis reaction and to form a powdery nitride product. PA1 I) how to supply sufficient nitrogen and how to make nitrogen to be mixed thoroughly with the metal powder; PA1 II) how to fully complete the reaction; and PA1 III) how to have the obtained powder to exist stably.
and EQU 3Si(NH).sub.2(s) .fwdarw.Si.sub.3 N.sub.4(s) +2NH.sub.3(g) ;
and
The direct nitridation method and the carbothermal reduction and nitridation method are both processed under high temperatures (e.g., from about 1000.degree. C. to about 1700.degree. C.) for long periods of time (e.g., from about 5 hours to about 10 hours) to fully complete the reaction. These method have the following disadvantages:
The imide decomposition method has disadvantages as follows:
The vapor-phase synthesis method has these disadvantages:
Compared with the aforementioned four methods, the combustion synthesis method is a new technique. It applies the self-propagating combustion reaction to synthesize ceramic materials, and has the following advantages:
The conventional techniques applying the combustion synthesis method for production of nitride ceramic powders include:
As mentioned above, using the combustion synthesis method, we have to overcome the following key problems:
According to the reported study, when some nitrides such as Si.sub.3 N.sub.4 and BN are to be synthesized, the pressure must be greater than 500 atm to start the reaction if nitrogen gas is used as the nitrogen source.
If liquid nitrogen is used as the nitrogen source (such as the Japanese PAT. PUBLICATION NO. 64-76906), the high pressure of N.sub.2 gas is not necessary. Whereas, this will result in a higher cost for the apparatus and operation, and in more complexity and danger during the operation.
When a solid-state nitride is used as the nitrogen source (such as the U.S. Pat. No. 4,459,363, the U.S. Pat. No. 4,944,930 and the Japanese PAT. NO. 64-76905), the high pressure of nitrogen gas is not necessary, either. While, in order that the reaction can proceed in a self-propagating combustion manner, the solid-state nitride must be easily decomposable and the reaction must be well-controlled so that the nitrogen produced by the decomposition can rapidly react with the powdery Al. Otherwise, there will be a resulting high pressure and the reaction is no more operative due to the fact that the nitrogen gas is escaping. Besides, a proper design for permitting the obtained powder to stably exist is also necessary.
It is therefore attempted by the Applicant to deal with the above situation encountered by the prior art.